Failed Shock Absorber Caused ATV Rollover?

Metallurgical failure analysis was requested of a failed rear shock absorber bracket from a four-wheel ATV involved in a fatal accident.

The failed shock absorber bracket was manufactured from a cast aluminum alloy. An overall view of a portion of the failed shock absorber bracket is shown in Photograph A.

Overall view of portion of failed shock absorber bracket.
Photograph A: Overall view of portion of failed shock absorber bracket.

The failed shock absorber bracket fracture surface shown in Photograph B contained a region of flat fracture, denoted with arrows. A profile view of the fractured shock absorber bracket, shown in Photograph C, reveals that the casting fracture is very straight and flat. The flatness of the fracture surface is denoted in Photograph C with arrows

Photo of flat fractured area of shock absorber bracket.
Photograph B:  Photo of flat fractured area of shock absorber bracket. Arrows denote area of flat fracture.

Profile view of flat fracture
Photograph C: Profile view of flat fracture. Arrows denote area of flat fracture.

Scanning electron microscopy (SEM) of the failed casting fracture surface was conducted in the areas denoted in Photograph D. The surface of the casting, at a magnification of 14x, revealed considerable casing shrinkage porosity. The casing porosity is shown in the scanning electron micrograph in Photograph E.

Denoted areas examined by Scanning Electron Microscope
Photograph D: Denoted areas examined by Scanning Electron Microscope (SEM).

SEM 14x
Photograph E: Scanning Electron Micrographs at magnification (14X) showing casting porosity.

A close-up view of the areas of casting porosity is shown in Photograph F. The characteristic appearance of shrinkage casting dendrites is clearly evident.

FSEM 200x
Photograph F: Scanning Electron Micrograph at higher magnification (200X) of casting porosity.

The fracture surface between the shrinkage porosity exhibited a dimple rupture fracture surface, and is shown in Photograph G. No evidence of metal fatigue (as preliminarily indicated by the flat fracture) was observed in the subject failed shock absorber bracket.

SEM 500x
Photograph G: Scanning Electron Micrograph of surface between shrinkage porosity exhibiting a dimple rupture fracture surface (500X Magnification).

From the above studies, it was evident that the failed ATV shock absorber bracket was not the cause of the subject accident. The dimple rupture fracture surface was the result of sudden impact, overload failure.

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Dr. R. Craig Jerner, Ph.D., PE specializes in accident investigation and metallurgical failure analysis, with over 30 years experience as a metallurgical consultant and accident investigator. He has testified as a metallurgical expert in over 250 depositions and more than 70 court appearances. If you or someone you know should need the services of  Dr. Jerner and J.E.I. Metallurgical, please visit our web site at the buttons below, or e-mail Dr. Jerner --- r.c.jerner@metallurgist.com


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